Chronic itch (pruritis) is a widespread condition that severely diminishes quality of life. However, there are few effective treatments for chronic itch, in part because the neural basis for itch remains poorly understood. Thus, the long-term goal of our research is to gain a better understanding of how itch is encoded in the nervous system at the level of specific neural circuits with the view of developing more effective therapies for pruritis. We previously discovered that the transcription factor Bhlhb5 is required for the surviva of a subset of inhibitory interneurons in the spinal cord (which are here termed B5-I neurons) that are required for normal itch sensation; mice lacking these spinal interneurons suffer from persistent pathological itch. These findings imply that B5-I neurons function to inhibit itch; however, the evidence is merely correlative. In this application we propose to use intersectional genetic strategies to manipulate the activity of B5-I neurons in order to establish cause-and-effect relationships between the activity of B5-I neurons and scratching behavior in mice. Here we propose to test this hypothesis through 3 specific aims: 1) Characterize the Bhlhb5-flpO knockin mouse, a key tool for our dual intersection strategy, and use this mouse together with the Ptf1a-cre line to genetically define B5-I neurons. 2) Investigate the functional response properties of B5-I neurons to natural stimulation of the skin and to confirm the ability of pharmacogenetic approaches to specifically manipulate their activity. 3) Use exocytogenetic and pharmacogenetic approaches to delineate function of B5-I neurons in vivo for itch-mediated scratching behavior. Results from these experiments will begin decoding the neural circuits that underlie itch by enabling us to visualize, characterize, and functionally manipulate B5-I neurons in vitro and in vivo. Moreover, the insight gleaned may have major clinical implications for people that suffer from chronic itch.